This invention relates to electrostatography and more particularly to a reproduction method and apparatus that employs transfers of toner images to and from intermediate transfer members.
In a multicolor electrophotographic (EP) reproduction apparatus, such as described in Tombs and Benwood, U.S. Pat. No. 6,075,965 including two or more single color image forming stations, a toner image is first electrostatically transferred from a moving primary image-forming member (PIFM), e.g., a photoconductor (PC), to a moving intermediate transfer member (ITM), and then subsequently electrostatically transferred from an ITM to a moving paper receiver sheet adhered to a transport web, employing a pressure transfer roller (PTR) located behind the transport web. The two toner transfers of each single color image take place in pressure nips respectively formed between PIFM and ITM, and between ITM and receiver sheet. The single color toner images from each of the two or more single color image forming stations are laid down one upon the other to produce, for example, a four-color toner image on a receiver sheet. In order to achieve a superior image quality, an important desire of a multicolor reproduction apparatus is good registration of the individual single color images on a receiver sheet. Moreover, it is highly desirable to minimize registration errors between individual single color images, such as may be caused by physical or mechanical effects associated with relative motions between the members.
As disclosed by Rimai et. al., U.S. Pat. No. 5,084,735 (1992), and by Zaretsky and Gomes, U.S. Pat. No. 5,370,961 (1994), a compliant intermediate transfer member (ITM) roller including a thick compliant layer and a relatively thin release layer improves the quality of electrostatic transfer of dry toner particles, as compared to the quality obtained using non-compliant intermediate transfer members, e.g., hard rollers. Not only is transfer improved from a primary image forming roller to a compliant ITM roller, but transfer is also much improved from the ITM roller to a receiver sheet.
Zaretsky, U.S. Pat. No. 5,187,526 (1993) discloses that electrostatic transfer of toner from an ITM roller to a receiver can be improved by separately specifying the electrical resistivities of the ITM and the TBR.
Bucks et. al., U.S. Pat. No. 5,701,567 (1997) describe an ITM roller having electrodes embedded in a compliant blanket to spatially control the applied transfer field.
May and Tombs, U.S. Pat. No. 5,715,505 (1998) and U.S. Pat. No. 5,828,931 (1998), describe a compliant imaging member including a thick compliant blanket coated with a thin photoconductive material.
The above mentioned patents describe benefits of using transfer rollers including a compliant layer. However, when one or more compliant ITM roller is used in an apparatus employing serial transfers of individual color toner images in succession to a receiver sheet, accurate registration in a resulting multicolor print can be more difficult to achieve.
It is well known that pressure nips formed by frictionally driven rollers coated with elastomers can exhibit a phenomenon known as overdrive (speed variations induced by strain changes). Compression of a solid elastomeric coating on a roller in a pressure nip produces a strain that changes the circumference of the roller, generally resulting in a changed surface speed in the nip. In the case of two elastomerically-coated rollers each of a different elastomer, with one roller frictionally driving the other, the result is to change the rotation rate of the driven roller as compared to its rotation rate in the hypothetical situation in which both rollers are nondeformable.
It will be evident that in a multicolor EP reproduction apparatus, e.g., such as described in U.S. Pat. No. 6,075,965 cited above, there will be variations in the precision with which the different rollers in the two or more single color image forming stations can be manufactured. There will also be variations in the precision of mounting of the roller members in the apparatus. Unless costly precautions are taken for manufacturing the rollers and for providing precision mechanisms for mounting them, even small variations will tend to produce significant differences in the amounts of overdrive developed by the various pressure nips in the apparatus. These differences can produce noticeable flosses of overall registration between different single color toner images on a receiver. The degree of strain induced speed variation in a pressure nip is also dependent upon the engagement, the strain generally tending to become greater as the engagement is increased. Speed variations associated with sets of elastomerically-coated rollers, such as rollers disclosed in U.S. Pat. No. 6,075,965 cited above, can vary due to changes of engagement associated with the flexing of rollers, thermal changes, drag force fluctuations, vibrations, and so forth. Moreover, variations in overdrive, sometimes referred to as xe2x80x9cdifferential overdrivexe2x80x9d, can occur along the length of a given transfer nip. Differential overdrive can be caused, for example, by local changes in engagement produced by variations of dimensions of the members forming a transfer nip, e.g., by roller runout, or by lack of parallelism of roller axes. Runout is defined here as the maximum radius measured from the axis of rotation of the core member minus the minimum radius measured from the axis of rotation of the core member, as measured over the entire operational length of the substantially cylindrical portion of the core member. Differential overdrive can result in a locally variable degree of slippage between rollers, which can produce image artifacts, including localized areas where a transferred toner image is distorted. Distortions resulting from differential overdrive in individual color separation toner images transferred sequentially to a receiver can produce a final multicolor print in which there may be localized patches in which registration is not optimal for all the colors.
In order to achieve very high quality color rendition including excellent registration in all areas of a print, it is necessary to provide improved means for controlling variations of overdrive from station to station in a color reproduction apparatus, and also to provide means for controlling differential overdrive. The present invention discloses a means for accomplishing these goals, by providing an improved compliant intermediate transfer roller for which the surface velocity in a transfer nip has a reduced sensitivity to external changes, such as: engagement, tension, drag forces, temperature, vibration, and the like.
The invention is directed to providing improved intermediate transfer member rollers in a multicolor electrostatographic apparatus which utilizes successive, image forming stations, each station providing a first electrostatic transfer of a toner image from a primary-image forming member to an intermediate transfer member and a second electrostatic transfer of the toner image from the intermediate transfer member to a receiver member carried through the image forming stations on a paper transport web, the intermediate transfer members having an improved structure to minimize station-to-station variability of overdrives associated with the first and second electrostatic transfers in each station.